Palladacycle is a cyclic compound having a palladium atom as a ring member. Palladacycle compounds, containing one or more heteroatoms, such as phosphorous, nitrogen, and sulfur, have been currently developed and used as catalysts to promote coupling reactions (e.g., the Heck reaction, the Suzuki reaction, and the Sonogashira reaction). Dupont, J., et al. Eur. J. Inorg. Chem. 2001, 15, 1917–1927; Herrmann, W. A., et al. Angew. Chem. Int. Ed. Engl. 1995, 34, 1844–1848; Beller, M., et al. Angew. Chem. Int. Ed. Engl. 1995, 34, 1848–1849; Herrmann, W. A., et al. J. Mol. Catal. A. 1996, 108, 51–56; Herrmann, W. A., et al. J. Organomet. Chem. 1999, 576, 23–41; and Reetz, M. T., et al. Angew. Chem. Int. Ed. Engl. 1998, 37, 481–483.
More recently, particular attention has been drawn to palladacycle catalysts that can be isolated from reaction mixtures and reused. Bergbreiter, D. E., et al. J. Am. Chem. Soc. 2000, 122, 9058–9064; Bergbreiter, D. E., et al. J. Am. Chem. Soc. 1998, 120, 4250–4251; Bergbreiter, D. E., et al. J. Am. Chem. Soc. 1999, 121, 9531–9538; Bergbreiter, D. E. Catal. Today 1998, 42, 389–397; and Nowotny, M., et al. Chem. Commun. 2000, 1877–1878. However, recyclable catalysts reported so far are not satisfactory. For example, they have low efficacy.